Hitherto, LANs (local area networks) with a bus type structure using coaxial cables, such as CSMA/CD (IEEE 802.3) type LANs and token bus (IEEE 802.4) type LANs, have been widely used. Recently, bus type networks using optical fibers and optical star couplers as transmission media have been studied.
These bus type LANs are characterized, like a ring type LAN, by freedom from the possibility that turning off the power supply of a node station may exert an influence on the total system. The bus type LANs, however, have the drawback that the total system runs down or the system must be stopped when the transmission line is broken or at the time of providing an additional station. In the ring type LAN, on the other hand, duplexing the transmission line and providing a loopback function will prevent the system from running down upon breakage of the transmission line at one place.
Concerning a bus type network, therefore, attempts have been made to duplex the transmission line and modem portions so that normal operation of the system will be secured even upon breakage of the transmission line or failure in a modem.
FIG. 8 shows a conventional duplexed structure in a bus type LAN which employs coaxial cables. Modems 11A, 11B, 11C and modems 21A, 21B, 21C are connected to a main transmission line 10 and a stand-by transmission line 20 through branching devices 13 and 23 and node stations 3A, 3B and 3C can selectively connect themselves to either the main transmission line 10 or the stand-by transmission line 20 by their own switches 4B, 4B and 4C, respectively. Designated by numerals 12 and 22 are terminating resistors, each of which has an impedance equal to the characteristic impedance of the transmission line.
FIG. 9 shows a duplexed structure in a broadband bus type network. A main transmission line 10 and a stand-by transmission line 20 are terminated on one end side thereof respectively by terminating resistors 12 and 22, each having an impedance equal to the characteristic impedance of the transmission line. Head end remodulators 41 and 42 are provided at the other ends of the transmission lines, and two-way amplifiers 43 and 44 are provided at intermediate points of the transmission lines.
Normally, node stations 3A to 3C are connected to main modems 11A to 11C through switches 4A to 4C thereof and perform data transmission using the main transmission line 10. That is, in normal state, data output from a data terminal of one of the node stations 3A, 3B and 3C, for instance, the node station 3A is transmitted through the main modem 11A and the main transmission line 10 to the other node stations 3B and 3C.
In FIGS. 8 and 9, if a certain mode, e.g., 11A fails, the respective switches 4A, 4B and 4C at the stations are simultaneously set to the dotted line side to resume the data transmission through the modems 212A, 21B and 21C and the stand-by transmission line 20.
When the main transmission line 10 is broken, for instance, between the modem 11A and the modem 11B as indicated by the mark "X" in FIG. 8, the switches 4A to 4C are changed over to connect the stations 3A-3C to the stand-by modems 21A-21C, whereby data transmission can be performed by use of the stand-by transmission line 20.
FIG. 10 shows a duplexed bus type LAN using optical fibers and optical star couplers as transmission media. A main transmission line 10 and a stand-by transmission line 20 are constructed by use of optical fiber cables 10a, 20a and star couplers 10b, 20b, and duplexed by providing modems 11A, 11B, 11C and modems 21A, 21B, 21C. Switching between the main and the stand-by system is performed by switches 4A, 4B and 4C.
The switching is carried out in the same manner as in the bus type LANs of FIGS. 8 and 9, that is, in the following manner.
In normal state, data output from a data terminal of one of the node stations 3A, 3B and 3C, for example, the node 3A is sent through the main modem 11A into the main star coupler 10a. In the star coupler 10a, the data is output evenly to each port, to be transmitted to the other node stations 3B and 3C. If a certain mode, e.g., 11B fails, the respective switches 4A, 4B and 4C at the stations are simultaneously set to the dotted-line side, to resume data transmission through the star coupler 20b, cable 20a and optical modems 21A, 21B and 21C belonging to the stand-by system.
The above-mentioned duplexed system according to the prior art, however, has following problems:
(1) Because failure in only one modem (e.g., 11A) causes switching of the modems at all the stations of the main transmission line to the corresponding ones of the stand-by system, the operation of the entire system is not ensured and a very dangerous condition results. That is, because the modems operating in normal conditions at other stations 3B and 3C than the station 3A are also switched to the stand-by modems the operations of which are not guaranteed, the reliability of the total system is not always improved by the duplexing. This applies also to the broadband type bus network; PA1 (2) A failure occuring in a modem or a drop cable for a station does not affect information transmission among the other stations, so that it is rather difficult for all the stations to recognize the failure. Without recognition of the failure, switching to the stand-by system cannot be achieved. To obviate such a situation, special information for verifying the operating conditions of each station must be transmitted; PA1 (3) Even if a transmission system for solving the problem in (2) above is adopted, it is still difficult to discriminate breakage of a power supply for a station and a modem failure from each other. As a result, some limitations are imposed on the greatest merit of the bus type network, namely, the merit that "the power supply at any station can be turned on and off freely"; and PA1 (4) Even after the main transmission line is broken, data transmission may be possible between the stations (e.g., 3B and 3C in FIG. 8) which do not have the breakage point therebetween. On the other hand, data transmission is impossible between the stations (e.g., 3A and 3C) having the breakage point therebetween. In this case, it is difficult for each of the stations to judge whether the power supply of the other station is turned off or the transmission line is broken. Thus, it is difficult for each station to detect a breakage of the main transmission line, by simply observing the data transmission condition.
It is accordingly one object of the present invention to provide a highly reliable duplexed bus type network which is capable of switching (changing over) only a portion associated with trouble.
It is another object of the present invention to provide a highly reliable duplexed bus type network which can easily detect breakage of a main transmission line and can switch the transmission line upon the detection.